A known variable valve timing control device for achieving a favorable driving condition by appropriately adjusting a valve timing of an internal combustion engine such as an automobile engine is described, for example, in JP2003-13714A (pp. 2-6, FIGS. 1-2) corresponding to U.S. Pat. No. 6,779,500 (Columns 2-10, FIGS. 1-2). The known variable valve timing control device includes a housing rotating in unison with a crankshaft, a rotor rotating in unison with a camshaft, a fluid pressure chamber provided between the housing and the rotor, a vane for defining the fluid pressure chamber into an advanced angle chamber and a retarded angle chamber, a lock mechanism including a lock plate configured to be projected from the housing into a lock groove formed on the rotor by means of supply of hydraulic oil for restricting relative rotation between the housing and the rotor at an intermediate phase between a most advanced phase angle and a most retarded phase angle, and an oil pressure control valve for controlling supply or discharge of hydraulic oil to the advanced angle chamber, the retarded angle chamber, and the lock mechanism. A passage for supplying or discharging hydraulic oil to or from the lock mechanism is formed independently from a passage for supplying and discharging hydraulic oil to and from the advanced angle chamber and the regarded angle chamber. Hydraulic oil is supplied to the oil pressure control valve from an oil pump driven by the internal combustion engine.
The variable valve timing control device moves a relative position of the vane in the fluid pressure chamber by supplying hydraulic oil to one of the advanced angle chamber and the retarded angle chamber and discharging hydraulic oil from the other of the advanced angle chamber and the retarded angle chamber to adjust relative rotational phase between the housing and the rotor. When the internal combustion engine starts up, for example relative rotational phase between the housing and the rotor is fixed by the lock mechanism, and after a warm-up period, the lock can be released by retracting the lock plate from the lock groove by supplying hydraulic oil to the lock mechanism. In this case, influence of pressure fluctuation is not likely to be transmitted to the passage for supplying or discharging hydraulic oil to the lock mechanism even if pressure of hydraulic oil in the fluid pressure chamber is fluctuated because of fluctuating torque from the camshaft when changing the relative rotational phase between the housing and the rotor by forming the passage for supplying or discharging hydraulic oil to the lock mechanism independently from the passage for supplying or discharging hydraulic oil to the advanced angle chamber and the retarded angle chamber. Thus, according to the known variable valve timing control device described in JP2003-13714A, when displacing relative rotational phase between the housing and the rotor, the device helps prevent that the pressure of hydraulic oil supplied to the lock mechanism is temporarily reduced by the pressure fluctuation due to a fluctuating torque from the camshaft and the lock plate is engaged in the lock groove at the intermediate phase.
However, with the known construction of variable valve timing control device described in JP2003-13714A, because a common oil pump is used for supplying hydraulic oil to the advanced angle chamber and the retarded angle chamber and for supplying hydraulic oil to the lock mechanism, pressure of hydraulic oil supplied to the lock mechanism descreases temporarily because a large amount of hydraulic oil needs to be supplied to the advanced angle chamber or the retarded angle chamber when the change in the relative rotational phase between the housing and the rotor needs to be large. Therefore, the lock plate may improperly engage with the lock groove at the intermediate phase, or the lock plate may be temporarily caught at the lock groove even if it is not fully engaged A need thus exists for a variable valve timing control device which prevents a lock mechanism from being caught in the lock groove upon a change of a relative rotational phase between a driving side rotational member and a driven side rotational member.